1. Field of the Invention
The present invention relates to a control system for a ring type network system, and more particularly to a control system for a ring type synchronous optical communication network system in which a plurality of nodes are connected via a ring type transmission path.
2. Description of the Prior Art
A synchronous optical network (SONET) in which a plurality of nodes are connected via a ring type transmission path is known to the art. Each of the nodes is provided with a cross connecting unit and a path switching unit. The cross connecting unit serves to make a cross connection between desired paths. The path switching unit serves to connect, for example, a low data transmission rate network to a desired path.
A description will be given, with reference to FIG. 1, of a conventional SONET. The SONET shown in FIG. 1 is a network in which a plurality of nodes A, B, C are connected via a ring type optical transmission line 34. The nodes A, B, C have cross connecting units 31a, 31b, 31c, path switching units 32a, 32b, 32c, and controlling units 33a, 33b, 33c, respectively. It should be noted that, although the SONET shown in FIG. 1 is described with only three nodes for convenience, a practical SONET generally comprises more than three nodes.
Each of the nodes A, B, C transmits multiple optical signals to the optical transmission line 34 comprising a plurality of optical paths in either direction. A cross connection between desired paths is made in the cross connecting units 31a, 31b, 31c. A connection of each reception path R is switched in the path switching units 32a, 32b, 32c. In the figure, the path switching units 32a and 32c are switched so that a signal from the node B is received by the node A, and a signal from the node A is received by the node C. Transmission paths S are connected to the corresponding path of the respective nodes A and C, and signals supplied via the paths S are added to signals transmitted via the optical transmission line 34 in either direction. Accordingly, the nodes A and C can mutually make a communication via the transmission paths S and the reception paths R.
On the assumption that a problem such as interruption of the optical fiber has occurred at a position marked by X on the optical transmission lines 34 between the nodes A and C, an optical signal transmitted by the node B cannot reach the node A. The node B detects the fact that the optical path to the node A is interrupted, and transmits, accordingly, to the node C an alarm indication signal (AI signal) via the transmission side of the cross connecting unit 31b. Additionally, after the node A detects the fact that the optical signal is interrupted, an AI signal is also sent from the reception side of the cross connecting unit 31a to the transmission side thereof. Formats for the AI signal have been already standardized in the SONET technology.
When the controlling unit 33a detects the AI signal, the controlling unit 33a switches the connection of the reception path R to the normally functioning side (node C side) opposite to the side (node B side) from which the AI signal is received. Additionally, the controlling unit 33c of the node C switches the path switching unit 32c when the AI signal is received from the node B. In the figure, since the reception path was initially switched to the node A side, the connection is not switched so as to maintain the connection in the same state. Accordingly, the communication between the nodes A and C can be continuously performed even though there is an interruption on the optical transmission line 34.
In the above-mentioned SONET, an unequipment code signal (UNEQ signal) is sent to a path which is not currently used. Formats for the unequipment code signal have been already standardized in the SONET technology. The controlling units 33a, 33b, 33c of the respective nodes A, B, C recognize that the path via which the UNEQ signal is received is a path which does not require a cross connection with other nodes, and accordingly the controlling units 33a, 33b, 33c do not control the path switching units 32a, 32b, 32c when the UNEQ signal is received.
In a case that the SONET comprises a number of nodes, alterations, cancellations and additions for the cross connection may be applied at various locations. If an erroneous setting of the cross connection is made, the corresponding path may be determined to be out of order. For example, when establishing a path between the nodes A and C, cross connection for the node B is set, as shown by a solid line in FIG. 1, in the cross connecting unit 31b. If the cross connection between the nodes A and C is not established, as shown by the cross connecting unit 31b in FIG. 2, due to an erroneous cancellation or an erroneous initial setting, an UNEQ signal is sent from the node B to the node A because a path between the nodes A and C is not established.
Since the controlling unit 33a does not perform switching on the path switching unit 32a when an UNEQ signal is received, the reception path R between the nodes A and C is determined to be interrupted at the node B. That is, there may be a problem in that communication between the nodes A and C is not established. In such a case, one solution to eliminate the problem is to search for the erroneously set node from among a number of nodes. That is, a seeking operation is performed for the particular node sending the UNEQ signal. However, finding the particular node is extremely difficult and not a practical solution because there are a number of cross connections in each of the nodes, which results in that a large number of cross connections must be checked.